X-chromosome replication in early rabbit embryos.

is generally believed that the sex chromatin which is found in the cells of 'Lammalian Eemales represents a genetically inactive X chromosome (LYON 1961 ; RUSSELL 1961 ) iwhich is heterochromatic and is replicated out of synchrony with the other X (ATKINS, TAFT and DALAL 1962; GRUMBACH, MORISHIMA and TAYLOR 1963; OHNO and CATTANACH 1962). According to the single-active-X hypothesis, inactivation occurs during early embryonic development in mammalian females. The only direct evidence for the time of inactivation has been the appearance of the chromatin body in the embryo. In the mammals whose embryos have been studied, the sex chromatin is generally not seen until sometime during the blastocyst stage (PARK 1957; MELANDER 1962). To obtain further evidence regarding the time of inactivation, inactivation was directly studied using a second method, autoradiography. Studies were made on the differences between replication patterns of X chromosomes of early female embryos prior to chromatin body Eormation and of later embryos having chromatin bodies. The results of these studies are the subject of this paper.